The invention relates to a heating boiler with a substantially horizontal burner chamber provided with a burner for combustion of liquid or gaseous fuel. A boiler water space surrounds the burner chamber and a boiler return connection passes into the boiler water space. A thermostat probe projects into the boiler water space and an auxiliary heating channel extends in the longitudinal direction of the burner chamber, the auxiliary heating channel being disposed in the boiler water space above the burner chamber. The inlet end of the auxiliary heating channel opens into an outflow chamber which is delimited by a boiler separator wall, the outflow chamber thus serving as a connection between the exhaust outlet of the burner chamber and the auxiliary heating channel.
A conventional embodiment of such a heating boiler has a cylindrical burner chamber with a closed rear end and a front end connected to the outflow chamber. The burner chamber is disposed within the lower region of a narrow oval outer boiler body. Recently it was found advantageous to employ a space-saving box-shaped oblong hollow body as the auxiliary heating channel in the upper region of the outer boiler body instead of a plurality of pipes which require many weld connections inter alia to the boiler separator wall. In order to promote heat transfer, the hollow body is provided with, for instance, comb-like inner ribs which are intensely heated by flue gases streaming from the burner chamber through the outflow chamber into the hollow body. The heat is transferred to the walls of the hollow body which in turn transfers the heat to the boiler water. Heating boilers of this or similar construction have a boiler water space containing comparatively little water due to their compact construction and the small, space-saving boiler dimensions. This has led to the following problem due to the small amount of boiler water used.
The regulation of the temperature of the boiler water is controlled by a regulating thermostat which is set at the desired discharge temperature of the boiler water in the discharge pipe connection which usually leads into a mixer valve. The regulating thermostat turns off the burner once the desired temperature is reached and is aided by a limiter thermostat that is set at a higher temperature, usually at a temperature of 90.degree. to 95.degree. C. In most cases the heating boiler is also furnished with a hot water system which can be heated by the boiler water to produce hot service water. When the water in such system demands heat, be it when actuating the heating boiler to supply hot water for the heating unit or installation or be it in summmer when the heating boiler works only for the heating of the water in the hot water system, the regulating thermostat is overrun so that the temperature of the boiler water may rise up to the set temperature of the limiter thermostat in order to accelerate the heating of the water in the hot water system.
Safety precautions have demand recently that a safety limiter be provided when a limiter thermostat is used for the burner regulation. The safety limiter responds at a maximum temperature of 100.degree. C. and locks mechanically after turning off the burner so that the burner stays switched off until the safety limiter is manually opened. With heating boilers of relatively small boiler water content, it sometimes occurs that the temperature of the heating boiler rises so fast that its water obtains a substantially higher temperature than set on the thermostat. This happens in summer when the heating boiler is only needed to furnish the hot water system with hot boiler water, or also when the heating boiler is used in a transitional season when very little boiler water is discharged through the barely opened mixer valve into the heating unit and is flowing back into the heating boiler as cooled return-flow water. An after heating effect is caused by the large time lag of the conventional thermostats and considerable amounts of heat are stored in the indirect boiler heating surfaces which are transferred to the boiler water after the burner is turned off. The temperature of the boiler water is thus increased.
As a consequence of the aforementioned cases of boiler use where the burner regulation for the heating of the hot water system is controlled by the limiter thermostat, operating troubles occur again and again because the further rise in boiler water temperature after the turning off of the burner leads to an exceeding of the activation temperature of the safety limiter so that it must frequently be manually released.
In order to prevent these operating difficulties, users set the limiter thermostat as well as the regulation thermostat lower by at least the temperature increase attributable to the after heating effect, which may amount to 8.degree. C., for instance at 82.degree. to 85.degree., in order to assure that the safety limiter will not react due to the after heating effect, after the turning off of the burner at least when the temperature of the boiler water has risen only to the temperature at which the limiter thermostat was set. This, however, results in the disadvantage that, when the heating unit needs much heat and the mixer valve opens completely and the whole amount of the water circuit of the heating unit passes through the heating boiler, a lower boiler water discharge temperature is obtained in the boiler discharge pipe, a temperature which at most only corresponds to the lowered set value of the limiter thermostat. This disadvantage remains also when the regulation thermostat is set at a higher temperature, because even at normal regulation of the burner by the regulation thermostat, the limiter thermostat remains active and turns the burner off as soon as the water temperature of the boiler water reaches the 82.degree. to 85.degree. set by the limiter thermostat. Due to the fact that most heating plants are set for outside temperatures of -15.degree. C. (5.degree. F.) and a maximum discharge temperature of 90.degree. C. (194.degree. F.) a lower setting of the limiter thermostat leads practically to the disadvantage of too low hot water temperature on very cold winter days. This problem of overshooting the temperature due to the post or after heating of the boiler water could be remedied by the use of substantially faster reacting and more sensitive thermostats with smaller shifting differentials. On one hand such thermostats are not to be found in the trade, and furthermore, such thermostats if used would substantially increase the frequency of switching intervals (switching on and off of the burner) which definitely would impair the efficiency of the burner-heating boiler system and would also cause impure flue gases, thus preventing their acceptance in many countries because of the impairment in air quality.